(a) Field of the Invention
The present invention relates to a MOS type image sensor suitably used for a camera and a reading device.
(b) Description of the Related Art
The MOS (metal oxide Semiconductor) type image sensor can be fabricated in a standardized MOS process, different from a CCD image sensor requiring dedicated processes The MOS type image sensor attracting the public attention in these days has advantages that lower electric power consumption can be achieved by an operation with a lower voltage and a single electric power source, and peripheral logic elements and macroelements can be mounted on a single chip.
In FIG. 1, an exemplified layout of a pixel section of the conventional MOS type image sensor developed by E. R. Fossum et al. (SPIE, 1900, pp.2-14, 1993) is shown. In this layout, a MOS type image sensor includes a pixel array including a plurality of pixel areas, each of the pixel areas generating a signal charge by photoelectric conversion, a row selection line disposed for each row of the pixel areas for selecting the row of the pixel areas, a signal reset line disposed for each row of the pixel areas for resetting the row of the pixel areas to a reset level, and a signal output line disposed for each column of the pixel areas for outputting the signal charge and the reset level from the corresponding column of the pixel areas selected by the row selection line. The signal reset line xe2x80x9cRxe2x80x9d and the row selection line xe2x80x9cSxe2x80x9d made of the same interconnect material has substantially same width and thickness.
In each of the pixel sections, the signal charge generated by the photoelectric conversion in the pixel section is taken out by a transmission line xe2x80x9cTXxe2x80x9dby way of a transfer FET to an output line VOUT. The signal charge is taken out by raising the potential of the row selection line xe2x80x9cSxe2x80x9d to a high level to raise the gate potential of the transfer FET. The reset line xe2x80x9cRxe2x80x9d is activated when the row selection line xe2x80x9cSxe2x80x9d is maintained at the high level. After the resetting, a reference signal is taken out to the output line VOUT The irregularities among the pixel sections are cancelled by using the difference between the signal level by the signal charge and the reset level by the resetting.
In FIG. 2 showing the read timing of the conventional image sensor, the row selection line xe2x80x9cSxe2x80x9d is, at first, activated to a high level to take out a signal charge in a light-receiving section xe2x80x9cPGxe2x80x9d for reading. When the row selection line xe2x80x9cSxe2x80x9d is maintained at the high level, the signal reset line xe2x80x9cRxe2x80x9d is activated to a high level and the reset level is taken out for reading after the reading of the signal charge.
However, the conventional technique has the following problems. In the arrangement between the signal reset line xe2x80x9cRxe2x80x9d and the row selection line xe2x80x9cSxe2x80x9d in the conventional image sensor, the area occupied in the pixel lo section by the pair of the lines xe2x80x9cRxe2x80x9d and xe2x80x9cSxe2x80x9d increases to consequently reduce the area occupied in the pixel section by the light-receiving section xe2x80x9cPGxe2x80x9d, thereby reducing the sensitivity to light.
As shown in FIG. 2, further in the conventional image sensor in which the signal reset line xe2x80x9cRxe2x80x9d and the row selection line xe2x80x9cSxe2x80x9d having substantially same width and thickness are adjacent to each other, each of the lines may generate coupling noises 14 due to the capacitance between the interconnects at the time of the rise and the fall of the signal. Especially, when the signal reset is malfunctioned at the time of activating the row selection line xe2x80x9cSxe2x80x9d, the signal level subjected to the photoelectric conversion at the light-receiving section xe2x80x9cPGxe2x80x9d may be destroyed.
In view of the foregoing, an object of the present invention is to provide an image sensor in which a larger area for a light-receiving section can be secured by reducing an area occupied by lines in a row direction, and a signal subjected to photoelectric conversion at the light-receiving section is hardly destroyed.
The present invention provides a MOS type image sensor including a pixel array including a plurality of pixel areas, each of the pixel areas generating a signal charge by photoelectric conversion, a row selection line disposed for each row of the pixel areas for selecting the row of the pixel areas, a signal reset line disposed for each row of the pixel areas for resetting the row of the pixel areas to a reset level, and a signal output line disposed for each column of the pixel areas for outputting the signal charge and the reset level from the corresponding column of the pixel areas selected by the row selection line, wherein the row selection line and the signal reset line are stacked sandwiching therebetween a dielectric film.
In accordance with the present invention, the areas of the signal reset line and the row selection line occupied in the pixel section can be reduced to consequently increase the area of the light-receiving section occupied in the pixel section, thereby increasing the sensitivity to light. Further, when the resistance of the row selection line is higher than that of the signal reset line, coupling noises are mainly generated in the row selection side and seldom generated in the signal reset line side at the time of the rise and the fall of the signal. Accordingly, the signal level obtained by photoelectric conversion in the pixel section is not destroyed.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.